qom: Change object property iterator API contract

Currently the ObjectProperty iterator API works as follows:

  ObjectPropertyIterator *iter;

  iter = object_property_iter_init(obj);
  while ((prop = object_property_iter_next(iter))) {
     ...
  }
  object_property_iter_free(iter);

This has the benefit that the ObjectPropertyIterator struct
can be opaque, but has the downside that callers need to
explicitly call a free function. It is also not in keeping
with iterator style used elsewhere in QEMU/GLib2.

This patch changes the API to use stack allocation instead:

  ObjectPropertyIterator iter;

  object_property_iter_init(&iter, obj);
  while ((prop = object_property_iter_next(&iter))) {
     ...
  }

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[AF: Fused ObjectPropertyIterator struct with typedef]
Signed-off-by: Andreas Färber <afaerber@suse.de>